REPRODUCTION IN ANIMALS
R
EPRODUCTION IN ANIMALS
D
o you recall the processes of
digestion, circulation and
respiration which you have
studied in your previous classes? These
processes are essential for the survival
of every individual. You have also learnt
about the process of reproduction in
plants. Reproduction is essential for the
continuation of a species. Imagine what
would have happened if organisms had
not reproduced. You will realise that
reproduction is very important as it
ensures the continuation of similar
kinds of individuals, generation after
generation.
You have already learnt in your
previous class about reproduction in
plants. In this chapter, we shall learn
how reproduction takes place in
animals.
6.1 Modes of Reproduction
Have you seen the young ones
of different animals? Try to name some
of the young ones by completing
Table 6.1 shown in examples at S. No.
1 and 5.
You must have seen the young
ones of various animals being born.
Can you tell how chicks and
caterpillars are born? How are kittens
and puppies born? Do you think that
these young ones looked the same
before they were born as they do now?
Let us find out.
Just as in plants, there are two modes
by which animals reproduce. These are:
(i) Sexual reproduction, and
(ii) Asexual reproduction.
6.2 Sexual Reproduction
Try to recall reproduction in plants
which you studied in Class VII. You will
remember that plants that reproduce
sexually have male and female
reproductive parts. Can you name these
parts? In animals also, males and
females have different reproductive parts
or organs. Like plants, the reproductive
parts in animals also produce gametes
that fuse to form a zygote. It is the zygote
which develops into a new individual.
This type of reproduction beginning
from the fusion of male and female
gametes is called sexual reproduction.
Let us find out the reproductive parts
Table 6.1
S. No. Animal Young one
1. Human Baby
2. Cat
3. Dog
4. Butterfly
5. Hen Chick
6. Cow
7. Frog
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in humans and study the process of
reproduction in them.
Male Reproductive Organs
The male reproductive organs include a
pair of testes (singular, testis), two sperm
ducts and a penis (Fig. 6.1). The testes
produce the male gametes called
sperms. Millions of sperms are produced
by the testes. Look at Fig. 6.2 which
shows the picture of a sperm. Though
sperms are very small in size, each has
a head, a middle piece and a tail. Does
it appear to be a single cell? Indeed, each
sperm is a single cell with all the usual
cell components.
Fig. 6.1: Male reproductive
organs in humans
What purpose does the
tail in a sperm serve?
Fig. 6.3 : Female reproductive organs in
humans
Female Reproductive Organs
The female reproductive organs are a pair
of ovaries, oviducts (fallopian tubes) and
the uterus (Fig. 6.3). The ovary produces
Oviduct
Ovary
Uterus
Head
Middle piece
Tail
Fig. 6.2 : Human sperm
REPRODUCTION IN ANIMALS
67
Sperm
duct
Testis
Penis
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Boojho recalls that the size
of eggs in animals varies.
The egg may be very small
as in humans, much larger
as in hens. Ostrich egg is
the largest!
Fertilisation
The first step in the process of
reproduction is the fusion of a sperm
and an ovum. When sperms come in
contact with an egg, one of the sperms
may fuse with the egg. Such fusion of
the egg and the sperm is called
fertilisation (Fig. 6.5). During
fertilisation, the nuclei of the sperm and
the egg fuse to form a single nucleus.
This results in the formation of a
fertilised egg or zygote (Fig. 6.6). Did
Fig. 6.5 : Fertilisation
Fig. 6.4 : Human Ovum
Nucleus
Sperms
Ovum
Fusing
nuclei
The process of fertilisation is the
meeting of an egg cell from the mother
and a sperm cell from the father. So,
the new individual inherits some
characteristics from the mother and
some from the father. Look at your
brother or sister. See if you can recognise
some characters in them similar to those
of your mother or your father.
Fertilisation which takes place inside
the female body is called internal
fertilisation. Internal fertilisation
occurs in many animals including
humans, cows, dogs and hens.
Fig. 6.6 : Zygote
female gametes called ova (eggs)
(Fig. 6.4). In human beings, a single
matured egg is released into the oviduct
by one of the ovaries every month.
Uterus is the part where development of
the baby takes place. Like the sperm,
an egg is also a single cell.
you know that the zygote is the
beginning of a new individual?
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REPRODUCTION IN ANIMALS
69
Why do fish and
frogs lay eggs in
hundreds whereas a
hen lays only one
egg at a time?
Have you heard of
test tube babies?
Boojho and Paheli’s teacher once told
them in the class that in some women
oviducts are blocked. These women
are unable to bear babies because
sperms cannot reach the egg for
fertilisation. In such cases, doctors
collect freshly released egg and
sperms and keep them together for a
few hours for IVF or in vitro
fertilisation (fertilisation outside the
body). In case fertilisation occurs,
the zygote is allowed to develop for
about a week and then it is placed in
the mother’s uterus. Complete
development takes place in the uterus
and the baby is born like any other
baby. Babies born through this
technique are called test-tube
babies. This term is actually
misleading because babies cannot
grow in test tubes.
female lays hundreds of eggs. Unlike
hen’s egg, frog’s egg is not covered by a
shell and it is comparatively very
delicate. A layer of jelly holds the eggs
together and provides protection to the
eggs (Fig. 6.7).
You will be surprised to know that
in many animals fertilisation takes
place outside the body of the female.
In these animals, fertilisation takes
place in water. Let us find out how
this happens.
Activity 6.1
Visit some ponds or slow-flowing
streams during spring or rainy
season. Look out for clusters of
frog’s eggs floating in water.
Write down the colour and size of
the eggs.
During spring or rainy season, frogs
and toads move to ponds and slow-
flowing streams. When the male and
female come together in water, the
As the eggs are laid, the male
deposits sperms over them. Each
sperm swims randomly in water with
the help of its long tail. The sperms
come in contact with the eggs. This
results in fertilisation. This type of
fertilisation in which the fusion of a
male and a female gamete takes place
outside the body of the female is called
external fertilisation. It is very
common in aquatic animals such as
fish, starfish
, etc.
Fig. 6.7 : Eggs of frog
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Development of Embryo
Fertilisation results in the formation of
zygote which begins to develop into an
embryo [Fig. 6.8(a)]. The zygote divides
repeatedly to give rise to a ball of cells
[Fig. 6.8(b)]. The cells then begin to form
groups that develop into different tissues
and organs of the body. This developing
structure is termed an embryo. The
embryo gets embedded in the wall of the
uterus for further development
[Fig. 6.8(c)].
The embryo continues to develop in
the uterus. It gradually develops body
Fig. 6.8 : (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells
(enlarged); (c) Embedding of the embryo in the uterus (enlarged)
How could a single cell
become such a big
individual?
(b)
(a)
(c)
Uterus
Embedding
embryo
Ovary
Ovulation
Zygote
Uterus
wall
Developing
embryo
Though these animals lay
hundreds of eggs and release
millions of sperms, all the eggs
do not get fertilised and develop
into new individuals.
This is because the eggs and
sperms get exposed to water
movement, wind and rainfall.
Also, there are other animals in
the pond which may feed
on eggs. Thus, production
of large number of eggs and
sperms is necessary to
ensure fertilisation
of at least
a few of them.
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REPRODUCTION IN ANIMALS
71
parts such as hands, legs, head, eyes,
ears etc. The stage of the embryo in
which all the body parts can be
identified is called a foetus (Fig. 6.9).
When the development of the foetus is
complete, the mother gives birth to
the baby.
place inside the egg shell during this
period? After the chick is completely
developed it bursts open the egg shell.
In animals which undergo external
fertilisation, development of the embryo
takes place outside the female body. The
embryos continue to grow within their
egg coverings. After the embryos develop,
the eggs hatch. Y
ou must have seen
numerous tadpoles swimming in ponds
and streams.
Viviparous and Oviparous Animals
We have learnt that some animals give
birth to young ones while some animals
lay eggs which later develop into young
ones. The animals which give birth to
young ones are called viviparous
animals. Those animals which lay eggs
are called oviparous animals. The
following activity will help you
understand better and differentiate
between viviparous and oviparous
animals.
Activity 6.2
Try to observe eggs of the following
organisms – frog, lizard, butterfly
or moth, hen and crow or any other
bir
d. Were you able to observe eggs
of all of them? Make drawings of the
eggs that you have observed.
The eggs of a few animals are easy
to observe because their mothers lay
them outside their bodies. These are
examples of oviparous animals. But you
would not be able to collect the eggs of
a dog, cow or cat. This is because they
do not lay eggs. The mother gives birth
to the young ones. These are examples
of viviparous animals.
Internal fertilisation takes place in
hens also. But, do hens give birth to
babies like human beings and cows? You
know that they do not. Then, how are
chicks born? Let us find out.
Soon after fertilisation, the zygote
divides repeatedly and travels down the
oviduct. As it travels down, many
protective layers are formed around it.
The hard shell that you see in a hen’s
egg is one such protective layer.
After the hard shell is formed around
the developing embryo, the hen finally
lays the egg. The embryo takes about 3
weeks to develop into a chick. You must
have seen the hen sitting on the eggs to
provide sufficient warmth. Did you know
that development of the chick takes
Fig. 6.9 : Foetus in the uterus
Uterus
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Can you now give some more
examples of viviparous and oviparous
animals?
Young Ones to Adults
The new individuals which are born or
hatched from the eggs continue to grow
till they become adults. In some
animals, the young ones may look very
different from the adults. The life cycle
of frog is shown in Fig. 6.10.
Observe the different stages of frog
starting from the egg to the adult stage.
We find that there are three distinct
stages, that is, egg → tadpole (larva) →
adult. Don’t the tadpoles look so
different from the adults? Can you
imagine that these tadpoles would some
day become frogs?
The tadpoles transform into adults
capable of jumping and swimming. The
transformation of the larva into an adult
through drastic changes is called
metamorphosis. What about the
changes that we observe in our body as
we grow? Do you think we too undergo
metamorphosis? In human beings,
body parts similar to those present in
the adults are present from the time
of birth.
6.3 Asexual Reproduction
So far, we have learnt about reproduction
in some familiar animals. But what
about very small animals like hydra and
microscopic organisms like amoeba? Do
you know how they reproduce? Let us
find out.
Fig. 6.10 :
Life cycle of frog
(a) Eggs
(b) Early tadpole
(c) Late tadpole
(d) Adult frog
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REPRODUCTION IN ANIMALS
73
Activity 6.3
Get permanent slides of
hydra. Observe them using
hand lens or a microscope.
Look out for any bulges
from the parent body.
Count the number of
bulges that you see in
different slides. Also, note
the size of the bulges. Draw
the diagram of hydra, as
you see it. Compare it with
the Fig. 6.11.
Fig. 6.11 : Budding in Hydra
Fig. 6.12 : Binary fission in Amoeba
(a)
(b)
(c)
(d)
Dividing nucleus
Daughter amoebae
In each hydra, there may be one or
more bulges. These bulges are the
developing new individuals and they are
called buds. Recall the presence of buds
in yeast. In hydra too the new individuals
develop as outgrowths from a single
parent. This type of reproduction in
which only a single parent is involved is
called asexual reproduction. Since new
individuals develop from the buds in
hydra, this type of asexual reproduction
is called budding.
Another method of asexual
reproduction is observed in the
microscopic organism, amoeba. Let us
see how this happens.
You have already learnt about the
structure of amoeba. You will recall that
amoeba is a single-celled organism
[Fig. 6.12(a)]. It begins the process of
reproduction by the division of its
nucleus into two nuclei [Fig. 6.12(b)].
This is followed by division of its body
into two, each part receiving a nucleus
[Fig. 6.12(c)]. Finally, two amoebae are
produced from one parent amoeba
[Fig. 6.12(d)]. This type of asexual
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Story of Dolly, the Clone
Cloning is the production of an exact copy of a cell, any other living part, or a
complete organism. Cloning of an animal was successfully performed for the first
time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh,
Scotland. They successfully cloned a sheep named Dolly [Fig. 6.13 (c)]. Dolly was
born on 5
th
July 1996 and was the first mammal to be cloned.
(a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly
Fig. 6.13
During the process of cloning Dolly, a cell was collected from the mammary gland
of a female Finn Dorsett sheep [Fig. 6.13 (a)]. Simultaneously, an egg was obtained
from a Scottish blackface ewe [Fig. 6.13 (b)]. The nucleus was removed from the
egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep
was inserted into the egg of the Scottish blackface ewe whose nucleus had been
removed. The egg thus produced was implanted into the Scottish blackface ewe.
Development of this egg followed normally and finally Dolly was born. Though
Dolly was given birth by the Scottish blackface ewe, it was found to be absolutely
identical to the Finn Dorsett sheep from which the nucleus was taken. Since the
nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not
show any character of the Scottish blackface ewe. Dolly was a healthy clone of
the Finn Dorsett sheep and produced several offspring of her own through normal
sexual means. Unfortunately, Dolly died on 14
th
February 2003 due to a certain
lung disease.
Since Dolly, several attempts have been made to produce cloned mammals.
However, many die before birth or die soon after birth. The cloned animals are
many-a-times found to be born with severe abnormalities.
reproduction in which an animal
reproduces by dividing into two
individuals is called binary fission.
Apart from budding and binary fission,
there are other methods by which a
single parent reproduces young ones.
You will study about these in your
higher classes.
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